My brother and his friend both did that. I don't know who started it, but one of them said that he had a 3D Yoshi game for DS. The other one said he also had it. So they started talking about it all the time, both saying it was their favorite game, each not realizing that the other one was making it up, so they were each too afraid to call the other guy out on his bullshit. I would hear them talk about it practically every day. This is what they bonded over. It was why they were friends.
Best examples I have all revolve around Pokemon. But then half of those turned out to be true. Like the one kid who said he had a copy of Pokemon Green from Japan. We all thought he was bullshitting. Then one day he actually brought it. He also showed us the Missingno item duplication glitch. And now I feel like I must be "that guy" since I can't find any evidence that this is possible, but I swear to god when he first showed me that glitch, instead of getting Missingno from the coast of an island, he did some thing where a trainer had one and he caught the trainer's missingno. Maybe it was one of those things where you can get a trainer to spawn with the missingno glitch.
Later the Pokegods became a thing, and there was this one kid who would constantly go on about "Pikablu, the goddess of all Pokemon." Now we know Pikablu was a common thing and just Marill, but I don't know where he got "goddess" from. The anticipation for Gen 2 was quite a time to be alive.